THE HYDROGEN-BRIDGED RADICAL-CATION ENTER-DOT-CENTER-DOT-CENTER-DOT-O=C-OH-CENTER-DOT- A COMBINED EXPERIMENTAL AND THEORETICAL-STUDY OF ITSSTABILITY AND DISSOCIATION CHEMISTRY( )

Dissociative ionization of dihydroxyfumaric acid generated the hydroge n-bridged radical cation [H2O ... H ... O double bond C-OH.+, 1, as sh own by a combination of tandem mass spectrometric techniques (metastab le ion, collision-induced dissociation, and MSI MS/MS experiments) and computational chemistry (using ab initio MO and density functional th eories). This hydrogen-bonded radical cation is predicted by theory to be more stable than the isomeric ions [H2O ... HO2CH](.+), 2/2a, [H2O ... HCO2H](.+), 3, and H2O-C(OH)(2)(.+), 4/4a, while HC(OH)(3)(.+) is not stable at all. The heat of formation of the most stable conformer of 1 was estimated as 73 kcal mol(-1) (with an assigned uncertainty o f +/- 4 kcal mol(-1)) and its isomers 2, 2a, 3, 4 and 4a are calculate d (Becke3LYP/ 6-31G* + ZPE) to be higher in energy by 10.0, 9.4, 13.3 , 18.1 and 19.1 kcal mol(-1), respectively. Accordingly, the MP4SDTQI/ 6-31G*//MP2/6-31G** + ZPE relative energies of 2a, 4 and 4a are 11.5 , 19.2 and 21.3 kcal mol(-1). Mass spectrometric experiments on isotop ically labelled di[O-18]hydroxyfumaric acid indicated that two 1,5-hyd rogen transfers and sequential expulsions of CO lead, via an intermedi ate dihydroxyketene-water type of ion-neutral complex, to the hydrogen -bridged product ion 1. Unimolecular metastable dissociations of the l atter lead to the proton-bound ion [H2O ... H ... OCO](+) and to the h ydronium ion, H3O+, as well as, upon collisional activation, to the di hydroxycarbene ion, HOCOH.+. (C) 1997 Elsevier Science B.V.